An oil or gas well includes a bore extending into a well to some depth below the surface. The bore is lined with tubulars or casing to strengthen the walls of the bore. To further strengthen the walls of the bore, the annular area formed between the casing and the bore is typically filled with cement to permanently set the casing in the bore. The casing is then perforated to allow production fluid to enter the bore and to be retrieved at the surface of the well.
Typically, downhole tools with sealing elements are placed within the bore to isolate the production fluid or to manage production fluid flow through the well. For example, a bridge plug, a fracture plug, or a packer is placed within a bore to isolate upper and lower sections of production zones. Thus, by creating a pressure seal in the bore, these plugs allow pressurized fluids or solids to treat an isolated formation. These tools are usually constructed of cast iron, aluminum, or other alloyed metals, but have a malleable, synthetic element system. The plug or packer system can also be composed of non-metallic components made of composites, plastics, and elastomers.
Slips are a part of these downhole tools, such as plugs and packers, and the slips can also be composed of metallic or non-metallic components. However, metallic slips can cause problems during mill-up operations of the downhole tools in horizontal wells. As one solution to these problems, slip segments composed of composite material can be held on a mandrel of a downhole tool, such as a plug. These composite slip segments are typically held together with bands on the tool's mandrel until actuated to engage the surrounding casing downhole. Additionally, the composite slips segments can have inserts or buttons that are composed of metallic materials (e.g., tungsten carbide or the like) that grip the inner wall of the surrounding casing or tubular. Examples of downhole tools with slip segments with inserts are disclosed in U.S. Pat. Nos. 6,976,534 and 8,047,279.
FIG. 1A illustrates a fracturing system 10 having a composite plug 20 according to the prior art disposed in a bore. As shown, the system 10 can have at least one of the composite plugs 20 disposed within the casing 12 lining the bore.
Casing 12, as known in the art, is used to further strengthen the walls of the bore, and therefore the area formed between the casing 12 and the bore is typically filled with cement to permanently set the casing 12 within the bore. Also as shown, the casing 12 is perforated to allow production fluid to enter the casing 12 so the produced fluids can be retrieved at the surface of the well. The perforations 15 in the casing 12 are formed in formation zones 14 as shown. The formation zones 14 indicate zones where production fluid potentially exists. Accordingly, the casing 12 at these zones 14 is perforated in order to allow fluid to flow into the casing 12 and eventually to the surface.
FIG. 1B illustrates the composite plug 20 of the prior art in more detail. As shown, the plug 20 has a mandrel 22. As known in the art, the mandrel 22 is designed with a cylindrical hole (i.e., bore) through the center to allow for pressure equalization and well flow back prior to milling up the plug 20 after its use downhole. Also as shown, the plug 20 has uphole and downhole slip assemblies 24a-b, each having slip segments 30, inserts 34, and bands 32. The plug 20 also has uphole and downhole cones 26a-b, backups 28a-b, and a packing element 29.
Conventional composite slip assemblies 24a-b include multiple slip segments 30 disposed around the mandrel 22. Further, bands 32 typically hold the slip segments 30 in place, and the composite slip segments 30 include one or more metallic inserts 34 in order to engage the casing (12). During operation, the slip segments 30 move away from the mandrel 22 and compress the inserts 34 against the surrounding casing (12) when the plug 20 is compressed. Examples of the operation of conventional slip components of such a plug 20 are disclosed in U.S. Pat. No. 7,124,831, which is incorporated herein by reference in its entirety.
One method for forming composite slip segments 30 uses sheet molding compound (SMC). As is known, the sheet molding compound includes discontinuous fibers (e.g., glass filaments) impregnated with a composite matrix (e.g., resin). The composite matrix can include thermosetting resin, fillers, and additives such as initiators, inhibitors, thickeners, mold release agents, and the like.
The sheet molding compound is formed by forming the matrix as a paste layer onto conveyed carrier film. A doctor box (also known as a paste reservoir) places resin paste onto the carrier film (typically composed of plastic). Afterward, discontinuous fibers are distributed onto the paste layer as a mat of fibers. For example, the carrier film with the past layer passes under a chopper that releases strands of glass fiber. Long strands (more than 1″) of chopped glass fibers are released onto the layer of resin.
Another layer of resin is applied to hold all the fibers together, and another carrier film is added on the top layer. Subsequently, the sandwich layers are calendered to compact, impregnate, and wet the fiber bed by the paste to form a compounded sheet, which can then be matured to a malleable state.
The sheet molding compound can be made into larger composite parts using compression molding. When the sheet molding compound has matured enough for molding, for example, the compounded sheet is cut into pieces or charges, and the carrier films are removed. The pieces are then stacked together and put in a mold mounted on a steel die in a hydraulic or mechanical press, which is heated and applies pressure to the stacked charge to form the component.
This conventional molding method used to mold the composite slip segment 30 orients the fibers in parallel layers. According, such a molding process may enable the composite slip segment 30 to have a higher compressive strength, but the slip segment may have a low shear or tensile strength due to the orientation of the fibers.
Another method for forming composite slip segments 30 that uses sheet molding compound (SMC) is disclosed in U.S. Pub. 2008/0199642 to Barlow. A strip of sheet molding compound is wrapped into a seven layer roll, is placed in the mold with the axis of the roll along the length of the mold, and is compression molded at about 4000-psi pressure at a temperature of about 300-310 degrees Fahrenheit for twelve minutes. The mold is overcharged with the sheet molding compound so that a portion of the charge is squeezed out of the mold between the two pieces of the mold. When the slip body is removed from the mold, only grinding off of flashing is needed at the parting line. Granular abrasive is used on the slip body for gripping an internal surface of a tubular.
Of course, cast iron or other metallic slip assemblies 24a-b can be used in applications with increased loads, higher pressures, and higher temperatures. Such metallic slips assemblies 24a-b provide a higher overall strength for high pressure and temperature environments. However, at least one problem associated with such metallic slip assemblies 24a-b is that it is often less desirable to use such metallic components due to the time required to mill the components.
That is, plugs 20 are sometimes intended to be temporary and must be removed to access the casing (12). Rather than de-actuating the plug 20 and bringing it to the surface of the well, the plug 20 is typically destroyed with a rotating milling or drilling device. As the mill contacts the plug 20, the plug 20 is “drilled up” or reduced to small pieces that are either washed out of the bore or simply left at the bottom of the bore. Consequently, the more metal parts making up the plug 20, the longer the milling operation takes, and more damage to the milling equipment may result. Although, alternative solutions like fabric wrap resin infusion have also been used to remedy the mechanical properties of composite slips, this method has pressure and or temperature limitations that render it less suitable for some operations.
Accordingly, there is a need for non-metallic slip 24a-b components that will effectively have a high strength in all directions, not just have a high strength in one direction and a lower strength in another. The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.